The leader of the Islamic State militant group that controls tracts of Syria and Iraqhas accepted a pledge of allegiance from Nigerian Islamists Boko Haram, his spokesman said, calling on supporters to fight in Africa.

Boko Haram, which has killed thousands and kidnapped hundreds during a six-year campaign to carve out an Islamist state in northern Nigeria, pledged its allegiance last week, highlighting increased coordination between jihadi movements across north Africa and the Middle East.

"Our caliph, God save him, has accepted the pledge of loyalty of our brothers of Boko Haram so we congratulate Muslims and our jihadi brothers in West Africa," Islamic State spokesman Abu Mohammad al-Adnani said in an audio message, referring to his group's leader Abu Bakr al-Baghdadi.

Islamic State, an ultra-hardline offshoot of al Qaeda, has declared a caliphate in captured territory in Iraq and Syria. The militants have gained global notoriety for killing or kidnapping members of ethnic and religious minorities and posting videos of its members killing Arab and Western hostages.

In the audio message, Adnani urged Muslims who could not join Islamic State in Iraq and Syria to enter combat in Africa instead, saying Boko Haram's pledge had opened a "new door for you to migrate to the land of Islam and fight."

"We are calling you up for jihadis, go."

US officials said they were skeptical of whether Boko Haram's allegiance to Islamic State, also known as ISIL, will translate into attempts by the Syria-based militants to provide the Nigerian movement with financial support or weapons.

"We do not currently see any reflections that ISIL would seek to subsume Boko Haram into its own caliphate or even prioritize a terrorist partnership with the group," a US intelligence official said.

"Both groups, however, benefit from making the world think that the two deadliest terrorist organizations in history are working together more closely than they actually are," the official said.

Islamic State militants, who reject all but their own limited interpretation of early Sunni Muslim theology as heresy, also issued a threat to Jews and Christians.

"If you want to save your blood and money and live in safety from our swords ... you have two choices: Either convert or pay jezyah," he said, referring to tax for non-Muslims under Islamic rule.

"(Otherwise) you will soon bite your fingers with remorse."